diff --git a/apps/web/src/components/DiffPanel.tsx b/apps/web/src/components/DiffPanel.tsx
index 53de0fdb8dd2..7a7e274999bf 100644
--- a/apps/web/src/components/DiffPanel.tsx
+++ b/apps/web/src/components/DiffPanel.tsx
@@ -37,6 +37,7 @@ import {
   resolveDiffThemeName,
   resolveFileDiffPath,
 } from "../lib/diffRendering";
+import { PREFERRED_HIGHLIGHTER } from "../lib/syntaxHighlighting";
 import { areAllDiffFilesCollapsed, toggleAllDiffFiles } from "../lib/diffCollapse";
 import { useTurnDiffSummaries } from "../hooks/useTurnDiffSummaries";
 import { useProject, useThread } from "../state/entities";
@@ -960,6 +961,7 @@ export default function DiffPanel({
                     lineDiffType: "none",
                     overflow: wordWrap ? "wrap" : "scroll",
                     theme: resolveDiffThemeName(resolvedTheme),
+                    preferredHighlighter: PREFERRED_HIGHLIGHTER,
                     themeType: resolvedTheme as DiffThemeType,
                     stickyHeaders: true,
                     ...(loadDiffFiles ? { loadDiffFiles } : {}),
diff --git a/apps/web/src/components/DiffWorkerPoolProvider.tsx b/apps/web/src/components/DiffWorkerPoolProvider.tsx
index 3ec748c6bcb2..bcf90b118410 100644
--- a/apps/web/src/components/DiffWorkerPoolProvider.tsx
+++ b/apps/web/src/components/DiffWorkerPoolProvider.tsx
@@ -4,6 +4,7 @@ import * as Schema from "effect/Schema";
 import { useEffect, useMemo, type ReactNode } from "react";
 import { useTheme } from "../hooks/useTheme";
 import { resolveDiffThemeName, type DiffThemeName } from "../lib/diffRendering";
+import { PREFERRED_HIGHLIGHTER } from "../lib/syntaxHighlighting";
 
 export class DiffWorkerError extends Schema.TaggedErrorClass<DiffWorkerError>()("DiffWorkerError", {
   operation: Schema.Literals(["create-worker", "get-render-options", "set-render-options"]),
@@ -73,6 +74,7 @@ export function DiffWorkerPoolProvider({ children }: { children?: ReactNode }) {
       }}
       highlighterOptions={{
         theme: diffThemeName,
+        preferredHighlighter: PREFERRED_HIGHLIGHTER,
         tokenizeMaxLineLength: 1_000,
         useTokenTransformer: true,
       }}
diff --git a/apps/web/src/components/chat/MessagesTimeline.tsx b/apps/web/src/components/chat/MessagesTimeline.tsx
index af920c0d6156..de509a43363a 100644
--- a/apps/web/src/components/chat/MessagesTimeline.tsx
+++ b/apps/web/src/components/chat/MessagesTimeline.tsx
@@ -43,6 +43,7 @@ import {
   resolveDiffThemeName,
   resolveFileDiffPath,
 } from "../../lib/diffRendering";
+import { PREFERRED_HIGHLIGHTER } from "../../lib/syntaxHighlighting";
 import ChatMarkdown from "../ChatMarkdown";
 import {
   BotIcon,
@@ -2078,6 +2079,7 @@ function UserMessageReviewCommentCard({ comment }: { comment: ReviewCommentConte
               collapsed: false,
               diffStyle: "unified",
               theme: resolveDiffThemeName(ctx.resolvedTheme),
+              preferredHighlighter: PREFERRED_HIGHLIGHTER,
             }}
           />
         ))}
diff --git a/apps/web/src/components/files/FilePreviewPanel.tsx b/apps/web/src/components/files/FilePreviewPanel.tsx
index a8c364763c28..f98179027315 100644
--- a/apps/web/src/components/files/FilePreviewPanel.tsx
+++ b/apps/web/src/components/files/FilePreviewPanel.tsx
@@ -25,6 +25,7 @@ import { useClientSettings } from "~/hooks/useSettings";
 import { useTheme } from "~/hooks/useTheme";
 import { getLocalStorageItem, setLocalStorageItem, useLocalStorage } from "~/hooks/useLocalStorage";
 import { DIFF_SURFACE_THEME_UNSAFE_CSS, resolveDiffThemeName } from "~/lib/diffRendering";
+import { PREFERRED_HIGHLIGHTER } from "~/lib/syntaxHighlighting";
 import { cn } from "~/lib/utils";
 import { isPreviewSupportedInRuntime } from "~/previewStateStore";
 import { resolvePathLinkTarget } from "~/terminal-links";
@@ -670,6 +671,7 @@ function EditableFileSurface({
               onLineSelectionEnd: handleLineSelectionEnd,
               overflow: wordWrap ? "wrap" : "scroll",
               theme: resolveDiffThemeName(resolvedTheme),
+              preferredHighlighter: PREFERRED_HIGHLIGHTER,
               themeType: resolvedTheme,
               unsafeCSS: FILE_LINK_REVEAL_UNSAFE_CSS,
               onPostRender: handlePostRender,
@@ -1039,6 +1041,7 @@ export default function FilePreviewPanel({
                     disableFileHeader: true,
                     overflow: wordWrap ? "wrap" : "scroll",
                     theme: resolveDiffThemeName(resolvedTheme),
+                    preferredHighlighter: PREFERRED_HIGHLIGHTER,
                     themeType: resolvedTheme,
                     unsafeCSS: FILE_LINK_REVEAL_UNSAFE_CSS,
                     onPostRender: onFilePostRender,
diff --git a/apps/web/src/components/pullRequest/PullRequestCodeTab.tsx b/apps/web/src/components/pullRequest/PullRequestCodeTab.tsx
index b0e00d57cc61..d9f1e7af716d 100644
--- a/apps/web/src/components/pullRequest/PullRequestCodeTab.tsx
+++ b/apps/web/src/components/pullRequest/PullRequestCodeTab.tsx
@@ -42,6 +42,7 @@ import {
   resolveFileDiffPreviousPath,
   type RenderablePatch,
 } from "~/lib/diffRendering";
+import { PREFERRED_HIGHLIGHTER } from "~/lib/syntaxHighlighting";
 import { cn } from "~/lib/utils";
 import { createPullRequestDiffFileContentsLoader } from "~/lib/diffFileContents";
 import {
@@ -743,6 +744,7 @@ export function PullRequestCodeTab({
       lineDiffType: "none" as const,
       overflow: wordWrap ? ("wrap" as const) : ("scroll" as const),
       theme: resolveDiffThemeName(resolvedTheme),
+      preferredHighlighter: PREFERRED_HIGHLIGHTER,
       themeType: resolvedTheme,
       stickyHeaders: true,
       loadDiffFiles,
diff --git a/apps/web/src/components/settings/SettingsFontPreviews.tsx b/apps/web/src/components/settings/SettingsFontPreviews.tsx
index 57d714d2042d..856d85fb098a 100644
--- a/apps/web/src/components/settings/SettingsFontPreviews.tsx
+++ b/apps/web/src/components/settings/SettingsFontPreviews.tsx
@@ -5,6 +5,7 @@ import { terminalThemeFromApp } from "../ThreadTerminalDrawer";
 import { useTheme } from "../../hooks/useTheme";
 import { DISCONNECTED_COMPOSER_PLACEHOLDER } from "../../composerPlaceholder";
 import { resolveDiffThemeName, type DiffThemeName } from "../../lib/diffRendering";
+import { PREFERRED_HIGHLIGHTER } from "../../lib/syntaxHighlighting";
 import { GhosttyTerminalSurface } from "~/terminal/ghostty/surface";
 
 // The font previews are the real surfaces, not lookalikes: the composer's
@@ -79,7 +80,7 @@ function loadDiffPreviewHtml(theme: DiffThemeName): Promise<readonly string[]> {
   if (promise === undefined) {
     promise = preloadPatchFile({
       patch: DIFF_PREVIEW_PATCH,
-      options: { diffStyle: "unified", theme },
+      options: { diffStyle: "unified", theme, preferredHighlighter: PREFERRED_HIGHLIGHTER },
     }).then((results) => results.map((result) => result.prerenderedHTML));
     diffPreviewHtmlByTheme.set(theme, promise);
   }
diff --git a/apps/web/src/lib/syntaxHighlighting.ts b/apps/web/src/lib/syntaxHighlighting.ts
index 171725617e80..3be008ac2b79 100644
--- a/apps/web/src/lib/syntaxHighlighting.ts
+++ b/apps/web/src/lib/syntaxHighlighting.ts
@@ -1,11 +1,20 @@
 import {
   getSharedHighlighter,
   type DiffsHighlighter,
+  type HighlighterTypes,
   type SupportedLanguages,
 } from "@pierre/diffs";
 
 import { resolveDiffThemeName } from "./diffRendering";
 
+/**
+ * Always highlight with the Oniguruma WASM engine — the JS regex engine can
+ * backtrack catastrophically and hang the tokenizing thread. The shared
+ * highlighter is a first-caller-wins singleton, so every creation site must
+ * pass this value.
+ */
+export const PREFERRED_HIGHLIGHTER: HighlighterTypes = "shiki-wasm";
+
 const highlighterPromiseCache = new Map<string, Promise<DiffsHighlighter>>();
 
 export function getSyntaxHighlighterPromise(language: string): Promise<DiffsHighlighter> {
@@ -15,7 +24,7 @@ export function getSyntaxHighlighterPromise(language: string): Promise<DiffsHigh
   const promise = getSharedHighlighter({
     themes: [resolveDiffThemeName("dark"), resolveDiffThemeName("light")],
     langs: [language as SupportedLanguages],
-    preferredHighlighter: "shiki-js",
+    preferredHighlighter: PREFERRED_HIGHLIGHTER,
   }).catch((error) => {
     if (language === "text") {
       highlighterPromiseCache.delete(language);